Titanium carbide preparation

ABSTRACT

PREPARATION OF TIC FROM IMPURE TITANIUM OXIDE CONTAINING STARTING MATERIALS BY USING A CARBIDING PROCESS INVOLVING A CATALYZER, A BINDER, A CONTROLLED HEATING SPEED AND TEMPERATURE, WHEREBY PURE TITANIUM CARBIDE GRAINS HAVING HIGH COMBINED CARBON AND LOW FREE CARBON CONTENTS ARE OBTAINED AS ONE PHASE AND A METALLIC MATRIX SURROUNDING THE TITANIUM CARBIDE AS ANOTHER PHASE. THE METALLIC MATRIX IS READILY REMOVED FROM THE TWO PHASE BODY BY LEACHING, THEREBY RECOVERING THE PURE TITANIUM CARBIDE.

United States Patent 3,786,133 TITANIUM CARBIDE PREPARATION Shin-TangChiu, St. Lambert, Quebec, Canada, assiguor to Quebec Iron 8: TitaniumCorporation Fer et Titane du Quebec, Inc., Sorel, Quebec, Canada NoDrawing. Filed Sept. 11, 1970, Ser. No. 71,352 Int. Cl. C(llb 31/30 US.Cl. 423-440 Claims ABSTRACT OF THE DISCLOSURE This invention relates toa production of titanium carbide; more particularly, this inventionrelates to a process for preparing titanium carbide from impure titaniumoxide containing starting material such as Sorelslag, ilmenite,perovskite, impure rutile, etc., whereby titanium carbide of high purityand exceptionally low carbon content is obtained from the startingmaterial; this invention also relates to an intermediate productcomprised of titanium carbide dispersed or surrounded in a matrix of ametal mixture formed from some of the metal oxides in the tartingmaterial mixture, this material, as such, has a number of uses.

In the production of titanium carbide from titanium oxide, it has beenthe desideratum to obtain a highly pure titanium carbide having a lowfree carbon content when starting from a low grade titanium oxidematerial such as Sorelslag, ilmenite, perovskite, etc. These titaniumoxide containing materials contain a relatively high amount ofimpurities such as oxides of magnesium, calcium, aluminum, silicon andiron. In the titanium carbide production, removal of these impuritieshas been attempted by a number of processes.

In general, titanium carbide is produced by using a titanium oxide (TiOcontaining material previously mentioned as a raw material, grinding itto a certain particle size, admixing the same with a carbonaceousmaterial such a coke, carbon, finely divided and calcined petroleumcoke, sawdust, etc., shaping this mixture with the aid of a binder suchas pitch, molasses, etc. and reacting (carburizing) it at a temperaturegenerally in the range from 1400" C. to 3000 C. most prevalently in therange from 1400 C. to 2000 C. Although the binder materials contain anumber of constituents, the effect of these constituents on thecarburization reactions has been subject only to speculation.

In most processes, the heating is effected in a resistance type offurnace or furnaces of similar kind; and the product obtained from theseimpure starting materials generally is a form of crude titanium carbideadmixed with free carbon and a mixture of carbides formed from theimpurities. In turn, this crude titanium carbide starting material hasbeen used as a source of raw material for the production of titaniummetal or titanium dioxide.

If a pure titanium carbide material is sought which can be used as ahard material for making tools, abrasives, or high temperatureresistance elements, then titanium carbide is mostly produced by usingpure titanium dioxide as a starting material or using a purified gradeof titanium oxide ore such as purified rutile which consists of about98% of titanium dioxide.

3,786,133 Patented Jan. 15, 1974 A further requirement for producingpure titanium carbide is a carbon of high purity, finely ground andcalcined. Thus, low ash reducing agents such as petroleum coke areusually employed. Hence, the requirement for using highly pure startingmaterials in titanium carbide production has prompted a number ofattempts to obtain pure titanium carbide from a low grade titaniumbearing material. However, heretofore a method has not been discoveredwhich would produce pure titanium carbide from impure starting materialsas well as a purification process especially adaptable for obtaining TiCwith a high combined and a low free carbon and for ready separation andremoval of the impurities from titanium carbide.

Thus, a problem in the direct preparation of pure titanium carbide fromimpure starting materials has been that many oxides besides titaniumoxides are contained in these starting materials. These impurities areoxides such as magnesium, calcium, aluminum, silicon and iron. If thesematerials are reduced and carbided in a graphite resistance furnace bythe conventional processes, these oxides are carburized or carbidedalong with the titanium carbide. The final result is a multi-carbideproduct of low quality because of the undesirable propertie imparted bythese carbide impurities such as reduced hardness, melting point andstability, etc. Additionally, often large amounts of free carbon arepresent in the titanium carbide.

For use in the making of cutting tools and for use as a pure material,titanium carbide containing excessive amounts of the above-mentionedcontaminants is unsuitable. Hence, the impure titanium carbide productshave been used in the preparation of titanium metal or titanium pigment,thus requiring added processes to upgrade these starting materials. Fromthe above, it follows that in order to utilize more economically animpure titanium oxide containing starting material, no intermediateprocesses can be tolerated which result in a titanium carbide of lowquality, cause the formation of free carbon in titanium carbide, orresult in impurities incapable of removal from the starting material.

It has now been found that a highly pure titanium carbide can beproduced which has a high combined carbon content, a very low freecarbon content, and which uses as a starting material an impure titaniumoxide containing material.

Moreover, it has been found that the production of the highly puretitanium carbide can be achieved in a conventional graphite resistantfurnace without the need for modifying the same; furthermore, it hasbeen found that the product obtained in this particularcarburizationreduction process contains the impurities in a formespecially suitable for dissolving or leaching by means of a weaksulfuric acid and/or a Weak hydroflouric acid solution, the combinationof both acids is especially advantageous.

Thus, it has been found that the carbiding process if carried outaccording to the novel method in which the impurities aid in thepurification of titanium carbide and when employing a catalyst and abinder, produces an intermediate material capable of furtherpurification and especially adaptable for producing titanium carbideusable in applications requiring pure or high grade titanium carbide.

Accordingly, it has been found that. this up-grading of impure titaniumcontaining starting material can be achieved sucessfully when aformulated starting composition is rapidly heated to a reactioncondition which causes volatilization of some of the impurities, aformation of somewhat rounded titanium carbide particles and thereduction of the non-volatile impurities, and surrounding of thetitanium carbide grains by a secondary metal or matrix phase. Thissecondary phase or matrix is composed of a metal mixture or alloy ofimpurities in a form especially adaptable for removal by leaching aftergrinding the intermediate product. The reduced impurities are in a formwhich leaves these substantially in their metallic state. Additionally,this crude carbide containing the titanium carbide and secondary phaseor matrix material has also high electric conductivity, and a highmelting point. It can be used as a grinding material in grinding wheels,as electrodes, or as heat-resistant material, as well as for surfacehardening.

As the result of X-ray studies and microprobe analysis, the titaniumcarbide product obtained for example from Sorelslag shows no detectableimpurities within the titanium carbide grain; the secondary matrixconsists of reduced impurities such as metallic Fe, Al, Si and some Ti.Impurities such as Mg, 8, Mn, K, P, and Ca have been removed byvolatilization and/or distillation. vIt is also observed that thecontent of Al, Si, and Fe in the phase, reduced material has beenslightly decreased after the carburization step. In the reduced product,sodium chloride is present in very small amounts, e.g. 0.010% afterreacting at 2500 C. Instead of Sorelslag, starting materials similar toit can be used in this process such as the previously recited low gradetitanium oxide containing materials.

Although the exact mechanism by which the improved result has beenachieved is not known, it is postulated, for the sake of clarity but notas an inviolable rule, that the impurities are caused first tovolatilize and/or distill and then caused to form a metal matrix as theresult of an addition of an alkali chloride preferably sodium chloridewhich apparently catalyzes the carburization reaction as well as theimpurity conversion-reducing reaction. It is possible that this stepwisecatalyzed removal of impurities prevents the formation of carbides andaids in converting into a volatile product, those impurities which arereadily reduced at lower temperature and also aids in re ducing thoseimpurities which help in the refining of titanium carbide. Also, it ispossible that rapid heating causes the formation of a relatively highamount of CO gas at high temperature, a condition which favors theformation of high combined carbon in titanium carbide. Thus, in order toobtain a pure TiC when using a low grade TiO as the starting material,it has been found that a. sodium chloride catalyzer in combination withrapid heating rate and proper temperature produces the pure TiC grain.

Still further, if carburization is carried out in the presence of sodiumchloride, prolonged carburization does not cause an increase in freecarbon; however, prolonged carburization in the absence of sodiumchloride increases the free carbon content. The reduced and nonvolatileimpurities are converted into a secondary metal matrix phase which hasan apparent capability for refining the titanium carbide and forproviding a better medium for carbon diffusion and for its uniformdistribution.

To sum up, it is postulated that sodium chloride aids in removingimpurities from titanium carbide and also in the formation of thetitanium carbide. Thus, in the TiC production, according to the methodherein, a metal matrix is formed after a step-wise volatilization ofsome of the impurities. This mixed, multi-phase metal matrix containssubstantial amounts of iron, silicon and aluminum which appears to aidthe purification of titanium carbide. As a result of first co-fluxing ofthe distillable on volatile impurities, secondly creating a strongcarburizing atmosphere during titanium carbiding, and thirdly theformation of a metal alloy matrix which seems to soak up the impuritiesto provide better carbon diffusion and further enhances the separationof the titanium carbide phase from the secondary, metal phase, a highlypure titanium carbide is produced from an impure low grade titaniferousmaterial. Also, the interface between carbon and titanium dioxide isbetter because of the shrinkage of an extruded shape in the presence ofsodium chloride. As a result of these competing phenomena, the unleachedproduct is about 50% of the weight of the starting material. The matrixor secondary phase can be readily identified as the secondary phase is ametallic material which shows a whitish metal appearance whereas thetitanium carbide grains are grey. As a consequence of the discovery, andas an example, titanium carbide of 99% purity, having 19.6% of combinedcarbon and less than 0.15% free carbon, is obtained from a low gradetitanium dioxide material in a single step. The theoretical value ofmaximum combined carbon is 20.5%.

The process by which the improved results are obtained is carried out asfollows. Titanium oxide in the form of an impure grade material such asSorelslag, rutile, ilmenite, perovskite, is finely ground such that astarting material may have a mesh size from -100 mesh to -400 mesh.Graphite of about 100 mesh to about 400 mesh is admixed therewith andthen admixed with flour or other suitable binders such as tar, molasses,etc. Included in this mixture is an alkali chloride catalyst in anaqueous solution. As sodium chloride is the least expensive material, itconstitutes the desired catalyst.

A suitable range of the titanium containing material and carbon in thestarting mixture should be in the proper ratio to reduce the oxides andform TiC; the flour binder from 3% to 10% weight in the mixture and thealkali chloride from 0.5% to 2% Weight in the mixture.

An example of a suitable composition is of the following proportions:683 parts of ground Sorelslag, about 251 parts of carbon as graphite,about 56 parts of flour and 10 parts alkali chloride (as NaCl) and 204parts of water. Generally, Sorelslag consists of the constituents in thefollowing exemplary ranges.

TABLE I.SORELSLAG COMPOSITION The above mixture of Sorelslag, carbon,flour and sodium chloride is extruded as a rod or in any other suitableform. A rod of about 10 cm. diameter formed at 20 to 40 kg./cm. ofpressure has been conveniently employed. An extruded briquette was heldat C. for 3 hours, then reacted at 2200 C. to 2600 C. for about 10-15minutes with 20 to 50 minutes to heat up in a graphite inductionfurnace. Moreover, upon normal cooling the free carbon content does notappear to increase.

A continuous or a batch process may be employed. In a continuous processaccording to means well known in the art, care must be taken that therod or shape which is being reduced and carbided is self-supporting andnot friable. The amount of carbonaceous material added depends upon therequired stoichiometric proportion for forming the titanium carbide andfor reducing the metallic impurities. No excess of carbonaceous materialis provided. As a suitable solid carbonaceous material, graphite, coke,or petroleum coke is employed.

The carburization reaction is caried out in a neutral gas such as argonor helium, or in a reducing gas such as carbon monoxide, hydrogen, or,in vacuum.

As it has been mentioned "before, the rapid heating and the treatment atthe desired temperature for about 15 minutes for a cm. diameterbriquette is found to be especially suitable for producing thecarburized product. Alternatively, the carburization or carbidingreaction may be monitored by the conversion rate of titanium oxide tocarbide and the formation of the metallic matrix. It is preferred thatthe starting material is heated at a rapid rate such that the desiredtemperature is achieved not later than after 70 minutes after thestarting material has been introduced into a furnace. Generally, aheating rate of from 100 C./min. to 36 C./min. from the calciningtemperature upwardly is suitable, a preferred rate is 70 C./min. to 50C./min. It is postulated that the short reduction time increases thecombined carbon in titanium carbide and decreases the free carbon. Slowheating appears to decrease the combined carbon content in the product.

It has been found advantageous to carry out the titanium reduction andcarburization under a neutral atmosphere such as that provided by argon.A reducing atmosphere or vacuum may also be employed.

It has been found that with an increase in temperature, the amount oftitanium in the reduced product first increases and then decreases, withabout 2500 C.i100 C. olfering the best operating range. With increasedtemperature, the amount of free carbon is decreased proportionately.Also, with increased temperature, the amount of impurities such asmagnesium, calcium, sulfur and phosphorus have disappeared, while someof the iron, silicon and aluminum is vaporized. Thus, a better refiningeffect has been obtained at higher temperature which also aids involatilization of the impurities.

For example at a temperature of 2500 C. and after treating for tenminutes a mixture of Sorelslag and the carbonaceous material, thereaction product contains relatively large titanium carbide grains ofapproximately 0.05 millimeters in diameter having little porosity.However, if the temperature is increased to 2800 C. for the sametreatment time, the titanium carbide grains become larger and moreporous. Further, if the temperature exceeds 2700 C., the extrudedstarting material melts and picks up carbon (0.5 to 9%) from thegraphite crucible to form a eutectic. To remove the formed free carbon,it would be necessary to apply another, complicated purificationprocess.

Thus, the size of the grains can range from 0.01 to 0.2 millimeters indiameter and can be adjusted by adjusting the temperature. It has alsobeen found that if the treating time is varied, the grain size can bemade to vary accordingly.

As a suitable furnace, a resistance type, induction, arc, etc., furnacemay be employed. The product obtained in the carburization step iscrushed in a conventional manner to expose the matrix to the leachingmedium. Generally, a crushed material of a mesh size less than about 100mesh is suitable. However, the desired mesh size is usually based on thesize required for the final pure product. The carburized product and themetal matrix are leached at about 70 C. with an aqueous solution such as1% to 3% sulfuric acid H SO, preferably 2% H 80 or 0.5 to 1% hyrofluoricacid HF, preferably a mixture of the two acids. The best results havebeen obtained when mixing the two leaching solutions and using them onthe carbided product. These acids, in mixture, leach out the metallicmatrix without attacking the titanium carbide.

After the leaching has been effected as determined by residual amountsof metal, the pure titanium carbide grains are collected as a residue.The grains are of a fine spherical type of various sizes which may beground according to need. It is also observed that conventionallyproduced titanium carbide is usually crushed and ground which results inirregularly shaped grains.

According to this invention, highly pure titanium carbide is obtainedwhich, after the leaching step, by means of the leaching agentsillustrated above, contains from 0 to 0.2% free carbon and about 19.7%combined carbon and 79.5% titanium. Moreover, titanium carbide producedaccording to this invention has a hardness from about 3500 (50 g. load)to 4200 V.H.N. (no crack load) which is the standard hardness range fora pure titanium carbide. In distinction from the final product, thematrix (Fe-Si-Al) surrounding the TiC has a hardness from approximately900 (50 g. load) to 1300 V.H.N. After reacting, e.g. at 2500 C., thefinal matrix is composed of about 75% Fe, about 14% Si, and about 11% ofA1. The matrix does not contain any detectable or significant amounts ofcarbides of these metals. The matrix also helps in grinding the titaniumcarbide.

The following has been included herein to illustrate the invention.Sorelslag of the following composiiton has been used as finely groundstarting material.

A mixture of 683 parts of the above defined material sized atapproximately -320 mesh, 251 parts of finely ground graphite at 320mesh, 56 parts of flour at -320 mesh and 10 parts of NaCl was preparedby admixing these components with 204 parts of water. This mixture wasthen extruded in a rod form as an 8 cm. diameter briquette at a pressurefrom 20 to 40 kg./cm. The mixture may also be extruded or compacted indifferent size or forms as long as the shape permits the volatilizationof impurities at the desired temperature. The extruded rod was thentreated at 2500 C. for about 10 minutes in a graphite tube inductionfurnace. After this carburization step, a bright, metallic rod wasrecovered from the furnace which weighed about 50% less aftercarburization. After crushing to about a -2.5}L (F.S.S. No.) size, theproduct was leached in an acid solution of 2% sulfuric (ranging from 1to 2%) and 0.5% hydrofluoric acid (ranging from 1 to 0.3%). With a 1% H50 and 1% HF solution at 70 C. for 3 hours, the residue contained 79.3%Ti, 19.75% combined carbon, 0.05% free carbon, 0.1% oxygen, 0.04% Zr,0.5% V, 0.18% Fe, with Si, Mg, Ca, Al not detectable. With 2% H 50 atroom temperature for 24 hours, the residue contained 0.50% Fe, less than0.079% Si, less than 0.07% Al, non-detectable Mg, Ca, S.

With a 1% HF solution at room temperature for 24 hours, the residuecontained 0.15% Fe, less than 0.03% Al, non-detectable Si, Mg or Ca.

From the above analysis it is evident that a starting material such asSorelslag which contains the impurities such as magnesium oxide, calciumoxide, aluminum oxide, silica and iron oxides, when treated according tothe present invention, results in a titanium carbide containing onlytraces of magnesium, calcium, and silicon, and minor amounts ofaluminum, vanadium and iron. After leaching with sulfuric acid andhydrofluoric acid the residue contains no detectable amounts of aluminumor silicon. Purity of the titanium carbide products is alsoexceptionally good, i.e. from about 99 to 100%.

7 What is claimed is: 1. A process for producing highly purifiedtitanium carbide from impure starting materials containing a titaniumoxide consisting essentially of:

admixing with a titanium oxide containing material comprised of atitanium oxide, metallic iron, an iron oxide, aluminum oxide, magnesiumoxide, calcium oxide, and silicon oxide, an alkali chloride in an amountfrom 0.5 to 2% by weight and a stoichiometric amount of carbonaceousmaterial in reference to reducible metallic impurities and titaniumcarbide formed of graphite, coke, and petroleum coke and binder for thesame in an amount from about 3% to by weight to form a cohesive reactionadmixture; heating rapidly at a rate of 100 C./min. to 36 C./ min. saidadmixture at a temperature within the range from 2000 C. to the meltingpoint and for a period up to minutes at the final temperature;

volatilizing from said admixture volatile impurities during the heatingperiod;

reducing non-volatile impurities to form a reduced metal matrixconsisting essentially of Fe-Al-Si and carburizing said titanium oxideto form as one phase titanium carbide with high combined carbon contentclose to theoretical value and less than 0.2% carbon as free carbonimpurity in a single step, said titanium carbide being formed as anintimate mixture of titanium carbide grains surrounded by said metalmatrix as another phase;

grinding said mixture of titanium carbide and metal matrix;

leaching said mixture with an aqueous solution of sulfuric acid andhydroflouric acid; and

recovering highly pure high combined carbon, titanium carbide grains.

2. The process according to claim 1 and wherein the titanium oxidecontaining material is Sorelslag, the metal matrix is a mixture ofFe-Al-Si with incidental amounts of other impurities associated withsaid Sorelslag, said aqueous solution used for leaching is a 1 to 2%aqueous solution of sulfuric acid and 0.3 to 1% aqueous solution ofhydrofluoric acid.

3. The process according to claim 1 and wherein the leaching solution is1 to 20% aqueous sulfuric acid and 0.1 to 3% hydrofluoric acid in aproper quantity.

4. The process according to claim 1 and wherein the temperature at whichthe starting material is reduced is from 2200 C. to the melting pointand the period for heating is less than 20 minutes, after asubstantially uniform temperature has been achieved throughout thestarting material.

5. The process according to claim 1 wherein the volatile impurities areremoved as an eifiuent from a reaction zone.

References Cited UNITED STATES PATENTS 2,149,939 3/1939 Kinzie et a123208 A 2,018,133 10/1935 Kirchner 23208 A 966,399 8/1910 Higgins 23208A 3,078,149 2/1963 Barber 23208 A X 2,972,530 2/ 1961 Zimmerley 23208 AX 3,369,891 2/ 1968 Tarkan et a1 -123 MILTON WEISSMAN, Primary Examiner

